1. Field of the Invention
The present invention relates to image forming apparatus, more particularly, to copiers and printers forming images using a plurality of laser beams.
2. Description of Related Arts
In the conventional image forming apparatus using laser beams, scanning means scans an object surface (a photoconductor) with a plurality of laser beams emitted from a light source to form latent images, and a beam-pitch detector is provided between the light source and the scanning means for correcting a beam pitch (a distance between beams) (see Japanese Examined Patent Publication No. Hei6(1994)-94215, Japanese Unexamined Patent Publication No. Sho63(1988)217763, Japanese Unexamined Patent Publication No.Sho60(1985)-166916).
In such image forming apparatus, however, since the beam-pitch detector is placed between the light source and the scanning means, it is impossible to detect a change of the beam pitch which may occur in an optical path from the scanning means to the scanned surface due to thermal expansion of an optical system or other causes. Thus, the beam pitch cannot be sufficiently corrected, and sometimes normal images cannot be formed.